Aerial lifts have a platform or bucket for supporting a human. The platform is connected to a turret by a boom. The platform can be raised to an elevated position or lowered to the ground. In addition, the platform can be swung clockwise or counterclockwise by rotating the turret about a chassis. The boom is typically telescoping so as to allow extension and retraction of the platform.
In prior art articulated aerial lifts, the boom is articulated with respect to the turret by way of parallel arms. The arms are connected to the boom by a riser. Articulation allows a wider operating range (or envelope) for the platform than is available for unarticulated or stick boom lifts.
In aerial lifts, stability is important. An unstable lift can overturn, wherein the lift (platform, boom and all) literally falls over. Overturning endangers those persons located on the platform as well as those persons located on the ground and adjacent to the lift.
Stability of a lift must be achieved in all positions of the platform. Two of the most critical platform positions are referred to as the worst forward stability position and the worst backward stability position. In the worst forward stability position, the arms are fully raised and the boom is horizontal and fully extended. This position produces the largest overturning moment when the platform is in the forward position. Stability in the worst forward stability position is achieved by adding counterweights to the back end of the turret or to the chassis. The counterweights provide a stabilizing moment.
In the worst backward stability position, the boom is raised to its maximum elevation and is fully retracted. The arms are in their lowermost position. In the worst backward stability position, the platform is located near or even behind a vertical axis extending through the turret. The counterweights used to provide stability in the worst forward stability position no longer serve to counter balance the platform, but instead add to the weight of the platform. This produces an overturning moment. Stability for the worst backward stability position is achieved by adding counterweights to the turret or the chassis in the correct location.
Stabilizing both positions (worst forward and backward stability positions) determines the total weight of the lift. It is desired to minimize the total weight so as to reduce the cost of the lift and to allow use of the lift in areas where weight restrictions apply.
Overall lift weight could be reduced by not allowing the platform to reach the worst forward stability position because this is not a useful position. This could be done by using switches, sensors and valves to limit boom extensions below a certain angle. However, these devices could fail, allowing an operator to unwittingly reach an unstable position.